1. Field of the Invention
This invention pertains to the field of ceramic whisker fabrication and, more particularly, to a furnace system for use in growing ceramic whiskers in a continuous process.
2. Description of the Prior Art
Interest in ceramic whiskers and fibers has increased enormously in recent years, as consumers of such products have sought out ever increasing taxing applications for ceramics, metals and plastics. These applications have involved the use of materials such as ceramics at higher temperatures and under increasing stresses. Under such conditions, monolithic ceramics have the tendency to suddenly fail, due to their brittle characteristics. Developers of ceramic products have found that the introduction of a second phase for reinforcement has been particularly effective in improving the fracture toughness of ceramics. Such products are called ceramic matrix composites. The reinforcing material can be either short fibers or whiskers, long fibers, or clothmats. Tests have shown that such reinforcements act as a means to disperse the hairline fault or crack that is the initiation of a fracture failure of a composite.
This principle of load distribution from a fiber is well known for both ceramic and polymeric matrix composites. It also applies for metal matrix composites in a similar manner. Ceramic whiskers can be used for reinforcement of all three matrix structures--ceramic, polymeric and metallic.
The use of fibers for reinforcement of composites is well understood. Whiskers are also fibers except their length to diameter ratio (aspect ratio) is much shorter--10:1 to 1000:1. Whiskers are typically thought to be 0.1 to 100 microns in diameter. Short fibers overlap the whisker-to-fiber region where the aspect ratio can approach infinity.
Ceramic whiskers are used today for reinforcement of alumina for cutting tools and wear parts, and with aluminum for some structural components in aircraft. The use of silicon carbide, a popular ceramic whisker, has greatly increased the life of cutting tools up to 100 times over that of tungsten carbide. It is also used to a lesser extent for wear parts and reinforcement in aluminum composites in aircraft components. It is believed that the total US market for silicon carbide whiskers is 50,000-100,000 pounds per year at $200-$600 per pound. Other whiskers, while of small volume, can very from $100 to $1000 per pound.
Long term, it is believed that reinforcement of ceramics could be the basis for large volume use of ceramics as it solves the inherent disadvantage of ceramics--its high potential for sudden brittle failure. Ceramic reinforcement can be the basis for ceramic use in military armour on tanks, vehicles, aircraft, etc., ceramic rotors, valves, cylinder liners and complete all-ceramic engines, reinforcement of tall buildings and even bridges, if its cost can be brought down to compete with steel. At a lower cost it can be equally useful in metallic and polymeric composites.
Many kinds of ceramic materials can be made into whiskers. They include silicon carbide, alumina or sapphire, silicon nitride, diamond, aluminum nitride, to name a few. Alumina or sapphire whiskers are particularly interesting as silicon carbide whisker compositions cannot be used for cutting ferrous metals because silicon carbide whiskers will react with the iron. This is unfortunate as the cutting of stainless steels and irons represent over 90% of all cutting tool needs.
Initial commercialization of ceramic whiskers or short fibers came about as a by-product of the electric furnace silicon carbide process before the turn of the century. Most early-produced whiskers were of such variable quality that they were not accepted in the market place.
Later, developers worked at perfecting a whisker growing process that pyrolyzed rice hulls as the feed stock. The pyrolysis of silica with carbon represents the most significant commercial process for ceramic whisker manufacture today. Two feedstocks are used for this process: rice hulls or finely divided silica, and carbon. Some SiC whiskers were made commercially for a brief time in the early 1970's starting with the same raw materials, but the processing was quite different.
The pyrolysis process was described by Dr. J. F. Rhodes in "Silicon Carbide Whiskers", 88th Annual Meeting Abstracts, American Ceramic Society, Chicago, 1986. Basically rice hulls, the by-product from rice hulling, which is high in silica, are first cooked to remove some volatile compounds. This material is then charged to a periodic (batch) furnace where it is heated to 1600.degree. C. Under these pyrolysis conditions, silicon carbide whiskers will grow from the exposed surfaces of pyrolyzed material. The whisker growth will be irregular because of the nature of the surface upon which growth occurs and because the process is non-catalytic. The whiskers can also interlock and cluster.
Upon completion of the pyrolysis, the reactor mass is shredded, dispersed and then the whiskers are separated from the residual rice hulls, dried and the residual carbon burnt off. In this subsequent processing, some of the whiskers are damaged. Some residual ashes from the rice hulls and soil remain with the whiskers. In addition, the whiskers get tangled and clustered and some of the original hulls remain in the product. The nature of such processing leads to highly irregular whiskers, where clusters can substantially reduce the effectiveness of whiskers for their intended purpose of reinforcement of composites. There is a 10-30% physical loss of whiskers in this downstream processing.
An additional disadvantage to the conventional pyrolysis process is the fact that it produces very small diameter whiskers between 0.1-1.0 microns. It is generally recognized that whiskers in this size range with an aspect ratio of 10 or more to 1 can pass through the human air breathing system and become lodged in the lungs. It is also believed by most that these whiskers are inert to human fluids in the body, so that in time they can be the initiator of internal growth such as cancer as is the case with asbestos. While it has not been fully proven that such ceramic whiskers or short fibers are inert, producers of whiskers have assumed this to be the case so that they are employing procedures as if the ceramic whiskers are carcinogenic. Some manufacturers of ceramic whiskers will not sell whiskers by themselves. It is believed that, in part, this is because of its potential health hazard.
The alternate process for producing ceramic whiskers and fibers is to employ a catalyst to promote the growth of whiskers. In this latter case control over the whisker forming process is greatly improved. By the proper selection of the catalyst (usually a solid) together with its particle size, specific diameter whiskers can be produced. If all the catalyst particles are of the same size, then the whiskers will be all of the same diameter. This catalytic process is often described as the VLS process--vapor, liquid, solid. The feedstocks are introduced into the reactor as a vapor (V), they are liquified (L) on the surface of the catalyst (S). The whiskers proceed to grow off the liquid surface with a catalyst particle remaining at the outer tip of the whisker. This process is described in Chapter 13--"Whiskers", Handbook of Reinforcements for Plastics, Van Nostrand Reinhold Co. 1987, editors John V. Milewski and Harry S. Katz.
In addition to the inherent advantages of producing whiskers of high uniformity, by the proper selection of catalyst size, whiskers of larger diameter can be made. A study entitled "Asbestiform Fibers-Nonoccupational Health Risks" by the National Research Council, National Academy Press, 1984 pp. 1, 36, concluded that small fibers or whiskers less than approximately three microns in mean aerodynamic diameter can enter the human airways. If of sufficient length, they could lodge in the lungs. In the VLS process, to be described below, the minimum diameter itself will be far in excess of three microns mean aerodynamic diameters. The larger diameter whisker is also more useful for composite reinforcement.
In addition to the toxicological advantages of VLS whiskers, they are much stronger than whiskers made by the pyrolysis process. The incorporation of such whiskers into alumina matrix leads to higher fracture toughness of the resultant composite. Silicon carbide whiskers can also be useful for reinforcement of metallic and polymeric composites.
In spite of the very significant advantages for whiskers produced by the VLS process, it is not commercially practiced to any significant degree at this time primarily because the productivity with this technology is extraordinarily low. Recent manufacturers have produced VLS whiskers for a short period of time at a great cost and, consequently, could not find any continued market for long term usage.
Dr. John V. Milewski summarized the current state of the VLS process in "Growth of Beta-Silicon Carbide Whiskers by the VLS Process", Journal of Materials Science 20 (1985) pp. 1160-1166 and in his Chapter 13 on "Whiskers" in Handbook of Reinforcements for Plastics (cited above). Reference is made of depositing VLS whiskers upon a single moving belt or producing whiskers in a furnace where one or more silica bricks coated with carbon are used for the silicon monoxide source for silicon carbide whiskers. Other references refer to multiple single point sources for the SiO. Milewski also refers to the need for a vapor stream of SiO which, when combined with methane (CH4) can lead to the formation of SiC upon a molten supersaturated solution on the surface of carbon. From this supersaturated solution will grow the whisker with a catalyst particle at its tip. Such growth will lead to a uniform crystal of silicon carbide, if the level of impurities is low. Impurities in silica brick can end up in the SiC whisker product. It should be noted that in spite of many impurities in VLS whiskers, they are considerably purer than whiskers from the conventional pyrolysis processes. For many advanced or structural ceramic applications, high chemical purity materials are required in addition to excellent physical properties.
Shalek et al. in "Synthesis and Characterization of VLS-Derived Silicon Carbide Whiskers", Whisker- and Fiber- Toughened Ceramics, Proceedings of an International Conference, ASM, 7-9 June 1989, pp. 53-62 describes an in situ whisker generator process wherein a plurality of whisker growing plates are batch processed in a furnace. Shalek et al. lists many of the most critical process parameters that must be controlled to obtain uniform, reproducible yields of prime VLS whiskers. While the in situ processes of the type disclosed by Shalik et al. have served the purpose in producing prime whiskers, they have not proved commercially economical because the furnace throughput is low.
As such, those concerned with the development of whisker growing systems have long recognized that VLS whiskers are clearly superior to whiskers from non-catalytic pyrolysis processes, but suffer from low productivity per unit furnace volume. This leads to extraordinarily high capital investments and high operating costs per unit volume of whisker produced. The present invention overcomes these problems.